You'll need to leave the overall opacity of the button symbol at 100%, then modify the opacity of each state (up, over, down, etc.) individually. The individual states cannot override a symbol setting. For instance, if your symbol was set to 50% opacity, setting the over state to 100% would only give you 100% of 50% opacity.
The only way is to do it old fashioned way:-) No button symbol. Make separate images for each button. slice and export then code the rollovers using behaviors in Dreamweaver.